Before individualizing (dicing) the substrate into a plurality of element chips by a plasma-etching technique, a protective mask (a protective film) is formed to protect the elements regions. Although a resist film is typically used for the mask, a coated film of water-soluble resin may be used as described in Patent Document 1 (JP 2014-523112 A or U.S. Pat. No. 8,703,581 B2). When the coated film of water-soluble resin is used as the mask, it is advantageously removed by flushing it with water in an efficient manner.
In the meanwhile, the substrate contains a surface of high water-repellency and the element regions in which exposed electrodes and bumps are arranged forming the fine asperity thereon. When the water-soluble resin solution is applied on such a bumpy surface to form a coated film, the water-soluble resin solution may fail to cover the whole surface without reaching to the portions close to the fine asperity, and the water-soluble resin solution may flow on the surface before it is fully dried to form the protective film due to less volatility of water. As such it is difficult to apply the coated film across the bumps on the substrate surface. When the protective film is deficiently formed across the bumps, the substrate in the element regions may be adhered with debris caused in a laser grooving step of the protective film along the dicing regions that define the element regions, and may be damaged by a plasma exposure in a plasma-etching step thereof. Thus, the present disclosure suggests applying the water-soluble resin solution in every corner across the bumpy surface, so that the coated film is formed to cover the whole surface in a uniform manner regardless the asperity thereof.
In order to form the coated film on the substrate surface in a more uniform manner, it is desirable to quickly remove or dry the volatile components in the water solution. The substrate may be heated to accelerate removing or drying the volatile components of the resin solution. However, while the substrate is processed with a holding sheet such as a dicing tape adhered thereon, since the holding sheet has a heatproof temperature of 80 degrees C. or less (mostly 60 degrees C. or less) and a poor thermostability, the holding sheet cannot be heated to the temperature sufficiently to remove or dry the volatile components of the water solution. If the coated film is dried insufficiently, a mask burning or a mask deformation may be caused in the laser grooving step and the plasma-etching step.